The present invention relates to semiconductor vacuum chambers, and mores specifically to an improved method and apparatus for baking-out and cooling-down a semiconductor vacuum chamber.
Many semiconductor device fabrication processes such as physical vapor deposition (PVD), high density plasma (HDP) deposition, etc., employ high vacuum chambers (e.g., 10xe2x88x928xe2x88x9210xe2x88x929 Torr) to affect the deposition of thin films on a semiconductor wafer. To reach such high vacuum levels after a vacuum chamber has been vented to atmosphere (e.g., for maintenance, cleaning, etc.) and to prevent film contamination due to the desorption of moisture and other gaseous elements and compounds (i.e., potential contaminants) from the chamber""s interior surfaces (e.g., the chamber""s shield, wafer pedestal, etc.) during elevated temperature processing, the vacuum chamber""s interior surfaces must be heated to an elevated temperature (e.g., about 200xc2x0 C.) for a time period sufficient to desorb the potential contaminants (i.e., chamber bake-out). Improper chamber bake-out manifests itself in a degraded pre-process or xe2x80x9cidlexe2x80x9d chamber pressure (i.e., base pressure), an enhanced rate of pressure rise from the base pressure when the chamber""s vacuum pump is shut-off (i.e., rate of rise or xe2x80x9cRORxe2x80x9d), and poor deposited film quality (e.g., poor film resistivity), as described below with reference to FIG. 1.
FIG. 1 is a side diagrammatic illustration, in section, of the pertinent portions of a conventional high density plasma sputtering chamber 21. The sputtering chamber 21 contains a wire coil 23 which is operatively coupled to a first RF power supply 25. The wire coil 23 may comprise a plurality of coils, a single turn coil as shown in FIG. 1, a single turn material strip, or any other similar configuration. The wire coil 23 is positioned along the inner surface of the sputtering chamber 21, between a sputtering target 27 and a wafer pedestal 29. The wafer pedestal 29 is positioned in the lower portion of the sputtering chamber 21 and typically comprises a pedestal heater (not shown) for elevating the temperature of a semiconductor wafer supported by the wafer pedestal 29 during processing within the sputtering chamber 21. The sputtering target 27 is mounted to a water cooled adapter 31 in the upper portion of the sputtering chamber 21 so as to face the substrate receiving surface of the wafer pedestal 29. A cooling system 31a is coupled to the adapter 31 and delivers cooling fluid (e.g., water) thereto.
The sputtering chamber 21 generally includes a vacuum chamber enclosure wall 33 having at least one gas inlet 35 and having an exhaust outlet 37 operatively coupled to an exhaust pump 39 (e.g., a cryopump). A removable shield 41 that surrounds the wire coil 23, the target 27 and the wafer pedestal 29 is provided within the sputtering chamber 21. The shield 41 may be removed for cleaning during chamber maintenance, and the adapter 31 is coupled to the shield 41 (as shown). The sputtering chamber 21 also includes a plurality of bake-out lamps 49 located between the shield 41 and the chamber enclosure wall 33 for baking-out the sputtering chamber 21 as described below.
The sputtering target 27 and the wafer pedestal 29 are electrically isolated from the shield 41. The shield 41 preferably is grounded so that a negative voltage (with respect to grounded shield 41) may be applied to the sputtering target 27 via a DC power supply 43 operatively coupled between the target 27 and ground, and a negative bias may be applied to the wafer pedestal 29 via a second RF power supply 45 operatively coupled between the pedestal 29 and ground. A controller 47 is operatively coupled to the first RF power supply 25, the DC power supply 43, the second RF power supply 45, the gas inlet 35 and the exhaust outlet 37.
To bake-out the sputtering chamber 21, conventionally the bake-out lamps 49 are switched on between about 90% to 100% power when the chamber is at high vacuum. The pedestal heater (not shown) of the wafer pedestal 29 is set at about 200xc2x0 C., and the water supply to the adapter may or may not be shut-off. The chamber then is allowed to bake-out for about eight hours during which time degassed material will raise the chamber pressure.
For chambers in which titanium, titanium nitride or tantalum nitride are deposited, the above bake-out procedure is sufficient to produce a good base pressure (e.g., low 10xe2x88x928 Torr range), ROR (e.g., about 10 to 20 nTorr/min), and good deposited film quality.
The reason for the success of this bake-out procedure is that both titanium and tantalum are excellent gettering materials and, therefore, once deposited on the chamber surfaces during wafer processing, can absorb (or xe2x80x9cgetterxe2x80x9d) moisture and other gaseous elements and compounds from the sputtering chamber""s atmosphere. Typically, these gettered contaminants do not desorb, even during elevated temperature processing, so that the chamber""s base pressure and ROR are not affected by the gettered contaminants. As well, the gettered contaminants do not significantly affect deposited film quality. An eight hour bake-out, however, results in significant process downtime for the chamber being baked-out, as well as for processing equipment upstream and downstream from the processing chamber. Overall fabrication throughput thereby is greatly degraded by conventional bake-out techniques.
When the conventional bake-out procedure is employed within a chamber for copper deposition (e.g., a copper HDP chamber) the results are less satisfactory due to copper""s poor gettering properties. For instance, even after an eight hour bake-out, a copper HDP chamber can exhibit a high base pressure (e.g., low 10xe2x88x927 Torr), a rapid ROR (e.g., about 200 nTorr/min) and a poor deposited copper film quality (e.g., poor resistivity). Accordingly, a need exists for an improved bake-out method that can be performed more rapidly then conventional bake-out methods (e.g., so as to improve chamber throughput), and that sufficiently bakes out even a copper chamber.
A process related to and often used in conjunction with processing chamber bake-out is processing chamber cooling or xe2x80x9ccool-downxe2x80x9d. As chamber cool-down often is performed following high temperature processing or following chamber bake-out, and can result in significant process downtime for the processing chamber being cooled, as well as for processing equipment upstream and downstream from the processing chamber. For example, the time required to perform chamber maintenance and repair is initially determined by the temperature of the various chamber components which must be sufficiently cooled before handling. Opening a chamber at elevated temperatures exposes personnel to safety hazards and may result in oxidation and contamination of the chamber.
In order to mitigate the effects of contamination, chambers are typically cooled under high vacuum conditions. Because some processing chamber components are operated at temperatures in excess of 600xc2x0 C., cool-down time may be on the order of hours. The exact time required to reach a desired temperature depends on the chamber. For example, chamber components having high thermal conductivity (such as aluminum components) are capable of cooling more rapidly than components having low thermal conductivity (such as stainless steel components).
FIG. 2 shows a cooling curve for a typical ionized metal plasma chamber cooled according to current practice. The chamber was operated under normal conditions and then allowed to cool under vacuum. The temperatures of a clamp ring, a coil, and a shield were measured and recorded. For comparison, the temperature of the shield was measured in two locations, zero (0) degrees from the RF feedthrough and one hundred thirty-five (135) degrees from the feedthrough. Because significant oxidation can occur at temperatures at or above 100xc2x0 C., the desired temperature before opening the chamber is preferably below about 50xc2x0 C. As can be seen from FIG. 2, the time required for all components to reach the desired temperature is at least three (3) hours. Thus, the chamber remains idle and nonproductive during this cooling period plus the time required to perform the routine maintenance or repair, and to bake-out the chamber thereafter.
One attempt to cool a chamber (specifically, a Czochralski silicon growth chamber) is found in U.S. Pat. No. 5,676,751, entitled, xe2x80x9cRapid Cooling of CZ Silicon Crystal Growth System,xe2x80x9d by Banan et al. The approach disclosed therein involves disposing a porous insulating ring within the chamber and then saturating the ring with a gas. The gas is intended to improve the thermal conductivity of the insulating ring and to provide an annular cooling medium for efficient heat exchange. Because the cooling ring is believed to transfer heat more rapidly than other chamber components the overall cooling time is reduced.
However, such an insulating system requires entirely new chambers having enlarged capacities to accommodate the insulating ring. Further, the porosity of the ring makes it unsuitable for chambers wherein process gases are needed such as CVD chambers or wherein a plasma is used such as a PVD, a CVD, or an IMP chamber. In such chambers, the process and plasma gases would be absorbed by the ring and/or outgassed during lower vacuum conditions thereby upsetting the deposition process and contaminating substrates.
Therefore, there remains a need for an apparatus and method which provides rapid cool-down of a vacuum chamber and its components from an elevated temperature which protects the chamber from contamination and oxidation while also ensuring the safety of personnel. Preferably, such a method may be easily adopted by existing vacuum chambers.
To address the needs of the prior art a novel method and apparatus for baking-out and for cooling-down a vacuum chamber are provided. In a first aspect of the invention, rather than maintain the chamber to a low pressure, a dry inert gas (e.g., semiconductor grade argon, helium, nitrogen, etc.) which conducts heat from the vacuum chamber""s bake-out lamps to the shield and from the shield to the other parts within the vacuum chamber is introduced during chamber bake-out. The dense inert gas behaves as a conduction path between the bake-out lamps and the shield and between the shield and the chamber parts surrounded by the shield (e.g., the target, the coil, the pedestal, etc.) so that the shield and other parts are heated more rapidly and to a higher temperature than conventional bake-out techniques that are performed under high vacuum conditions. With use of the present invention, even copper chambers are sufficiently baked-out in a fraction of the time required to bake-out a chamber by conventional techniques. Applicants have found that the inert gas does not adversely become trapped in chamber components or later outgas, and due to the uniform heating of chamber components, contaminants desorbed from one chamber surface do not reabsorb on another chamber surface.
To bake-out a vacuum chamber the chamber is pumped out and is then isolated from the chamber""s vacuum pump. A volume of inert gas such as argon, helium or nitrogen is injected into the chamber, the chamber""s bake-out lamps are turned on and the cooling fluid flow to the adapter is turned off. The inert gas may be injected, the baking lamps may be turned on and the cooling fluid flow to the adapter may be turned off simultaneously or in any order.
Preferably, the amount of inert gas injected raises the chamber pressure to about 500 Torr (e.g., close to but less than atmospheric pressure).
Because of the rapid transfer of heat between the bake-out lamps and the shield and between the shield and the other chamber parts through the gas as a heat transfer medium, adequate chamber bake-out occurs quickly (e.g., typically in about two hours depending on the chamber involved, the pressure of the inert gas, the inert gas employedxe2x80x94gasses of smaller atomic mass conduct heat faster, etc.). After the chamber is sufficiently baked-out, the baking lamps are turned off and cooling fluid is flowed to the adapter so as to cool the inert gas before it is pumped from the vacuum chamber (e.g., to prevent overheating of the cryopump). Because the adapter and the shield are coupled, the adapter cools the shield, and the shield cools the inert gas. The inert gas is quickly cooled thereby (e.g., typically in about one hour). After the inert gas has cooled, it is pumped from the vacuum chamber, and the bake-out of the chamber is complete.
With use of the inventive bake-out method, chamber bake-out can be performed in far less than half the time of conventional bake-out techniques. Specifically, the present inventor have found that using the conventional bake-out technique described with reference to FIG. 1, the aluminum shield of a copper HDP chamber reaches only a temperature of about 120xc2x0 C. even for an eight hour bake-out. However, by employing the inventive bake-out method, the same aluminum shield can reach 200 to 300xc2x0 C. during bake-out. In fact, care must be taken not to melt the aluminum shield due to the rapid conduction of heat between the bake-out lamps and the shield. Accordingly, a highly improved bake-out method is provided.
In a second aspect of the invention, a process chamber is provided having at least one source of a cooling gas having a high thermal conductivity. The gas is input into the chamber and allowed to reside therein for a period of time. Once a target temperature is reached for the chamber and its components, the cooling gas is evacuated.
In another aspect of the invention, a cooling gas having a high thermal conductivity is input into a process chamber until a desired pressure is reached. The chamber is allowed to cool for a period of time and then the cooling gas is evacuated. During the cooling stage, a pressure equilibrium may be maintained by periodically flowing additional cooling gas into the chamber.
In yet another aspect of the invention, a cooling gas is charged into a process chamber until a desired pressure is established in the chamber. The cooling gas in brought into contact with chamber components to allow for thermal conduction therebetween. During the cooling stage, a pressure equilibrium is maintained in the chamber by providing a constant flow of the cooling gas into the chamber while simultaneously evacuating the chamber at a substantially equal rate by engaging a vacuum pump.
In still another aspect of the present invention, a process chamber is purged by a purge gas and a cooling gas is then input into the chamber. Thereafter, the cooling gas is evacuated, cooled, and returned to the chamber. The cooling gas is permitted to reside within the chamber for a period of time or, alternatively, continuously recycled. Any of the above cooling aspects may be used to affect more rapid cooling of a processing chamber following chamber bake-out.